Dear Dr. Irwin, I’ve read that for sunscreen to be effective, it has to be reapplied every 2 hours. I put on sunscreen every morning with my moisturizer, but I don’t go outside until the afternoon … so if it only works for 2 hours, wouldn’t it be ineffective by then? Would it be a better use of time to wait to apply sunscreen until I go outside? Thanks! Jessica Generally, sunscreen lasts longer than that. I think the manufacturers figure 1) people don’t apply enough to start with, 2) they might be sweating or in the water, and 3) it doesn’t hurt for them to sell more.
Since it’s really impossible to do real time, real people SPF studies, they do the UV determinations on plastic discs (UVA) and/or with little squares on human backs left exactly a certain amount of time and then soaking the person in water. These are better than no studies, but not very accurate for real life conditions.
No matter what the SPF says, as long as it’s 30 or over, zinc oxide in 10-20% concentrations is the most effective ingredient. See our sunscreens in the shop.
I think we need to distinguish between low, medium, and high sun/light conditions. So let’s use skiing and snowshoeing as an example!
Low sun conditions
Let’s say you get up in the morning, you’re planning to go skiing, but it’s cloudy, freezing and you decide not to go. You plan to go into town for groceries later in the day instead and read a book. Then apply your sunscreen in the morning, and unless you’re going to be outside walking for groceries for more than 15 min, you don’t need to reapply. Just wear a hat with a brim if you can.
Medium sun conditions
You get up in the morning and apply your sunscreen. You go skiing, and you’re wearing a face mask part of the time. But your body is covered. Skiing is at 7,000 ft, and it’s sunny so there’s reflection off the snow. After you come in for lunch, reapply the sunscreen before you go out, and it doesn’t hurt to carry a small stick sunscreen with zinc with you, in case you stay out longer.
High sun conditions
You get up in the morning an apply your sunscreen. It’s sunny, spring skiing, and you’re not wearing a face mask. The ski helmet doesn’t have a brim at all. You’re at altitude remember and there’s reflection! Best to apply sunscreen every two hours or so, especially if you’re sweating.
You are all smart, and the above will make sense. Each situation is a little different. Maybe I’ll do a post on sunny vacations….but wait Covid, so maybe next year!
Hope this helps,
Dr. Brandith Irwin
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